Rotational energy profiles for internal rotations about the central C-C bond of n-butane and 2,2,3,3-tetrafluorobutane (TFB) are calculated using ab initio and semiempirical quantum chemical methods. Extensive geometry optimizations are performed for each rotamer. The rigid rotor approximation is seen to provide an adequate representation in the transgauche transition region of n-butane but CCC bond angle optimizations are necessary in the vicinity of the syn conformer. Optimizations of several geometrical parameters are required for accurate results with the fluoro-substituted molecule. The approximate ab initio PRDDO method is in good agreement with minimal and extended basis set ab initio results for n-butane but furnishes poor energy differences and barriers for TFB. On the other hand, PRDDO does provide a cost-effective means of obtaining optimized geometries on which to perform ab initio calculations.

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Originally published in the Journal of the American Chemical Society by the American Chemical Society . Publisher’s PDF available through remote link. DOI: 10.1021/ja00531a010